In an LTE (Long Term Evolution) system, for a cell edge user, an SFBC (space frequency block coding) transmission mode is used to improve a cell edge signal-to-noise ratio. For a cell center user, a transmission mode of multi-layer parallel transmission is used to provide a relatively high data transmission rate. If a base station may obtain all or a part of downlink channel information, the base station may use a precoding technology to improve signal transmission quality or a signal transmission rate. For a TDD (time division duplex) system, a radio channel has uplink and downlink reciprocity, and a precoding weight vector of a downlink channel may be estimated based on an uplink channel. However, for an FDD (frequency division duplex) system, because an uplink carrier frequency and a downlink carrier frequency are different, a precoding weight vector of a downlink channel cannot be obtained by using an uplink channel.
In the LTE system, for the FDD system, the base station obtains a precoding matrix in a manner in which a terminal device feeds back the precoding matrix. Currently, feedback load is reduced by using a two-level codebook feedback mechanism, that is, the precoding matrix W is constituted by a product of a first-level feedback matrix W1 and a second-level feedback matrix W2:W=W1×W2  (Formula 1)
The first-level feedback matrix W1 is applicable to entire system bandwidth, and has a long period characteristic. A function of the second-level feedback matrix W2 is that each sub-band selects a precoding weight vector and selects a phase (co-phase) difference between two polarization directions based on the second-level feedback matrix. Because W2 is fed back on the basis of each sub-band and has a short period characteristic, a quantity of bits required for feeding back W2 largely affects feedback overhead.
With application of a two-dimensional (2D) antenna technology, a data throughput rate and edge coverage in both a horizontal direction and a vertical direction need to be obtained in a precoding manner. Currently, W1 is constructed in a commonly used manner of a Kronecker product of a beam vector in a vertical direction and a beam vector in a horizontal direction:
                              W                                                            ⁢            1                          =                  [                                                                                          X                    H                    k                                    ⊗                                      X                    V                    l                                                                                                                                                                                                                                                                                                        X                    H                    k                                    ⊗                                      X                    V                    l                                                                                ]                                    (                  Formula          ⁢                                          ⁢          2                )            
XHk is a beam vector cluster selected from the horizontal direction, and includes MH column vectors; XVl is a beam vector cluster selected from the vertical direction, and includes MV column vectors.
Because an increase in codebooks (codebook) results in an increase in overhead required for feeding back the codebooks, how to reduce the overhead for feeding back the codebooks is a problem that needs to be resolved. In the LTE Rel. 10 protocol, when a rank is 1 to 4, four bits are required to feed back W2. A degree of freedom in a vertical dimension is used in the 2D antenna technology. Therefore, there are more than four to-be-selected beam vectors in W2, and therefore not only complexity of selecting a beam vector by a receive end is increased, but also complexity of feeding back a selected codebook is increased.
To overcome the increase in feedback overheads caused by the use of the 2D antenna technology, an existing technical solution uses a downsampling (subsampling) method. Although the downsampling method can reduce the feedback overhead, a spatial resolution of the beam vector is reduced to some extent. However, in the solution, a same column vector is applied on different polarization antennas, and therefore beam coverage has different performance in different angles, thereby causing a coverage hole.
In conclusion, although the foregoing method can reduce feedback overhead, there is a defect of unbalanced beam coverage performance.